WO2013141726A1

WO2013141726A1 - Method of manufacturing synthetic esters

Info

Publication number: WO2013141726A1
Application number: PCT/PL2013/000031
Authority: WO
Inventors: Wacław KORZENIOWSKI; Łukasz KORZENIOWSKI; Krzysztof KORZENIOWSKI
Original assignee: Chemenergia Korzeniowski Wacław
Priority date: 2012-03-23
Filing date: 2013-03-12
Publication date: 2013-09-26
Also published as: PL227792B1; CN104024195B; PL398598A1; CN104024195A

Abstract

A method to manufacture synthetic esters from a liquid stream formed during a cyclohexane oxidation process and the liquid stream contains acids, peroxides, hydroxycaproic acid esters, esters of cyclohexaneodiols, lactones, products of cyclic compounds condensation, as well as water, and this method is characterized by that the liquid stream undergoes an initial process of reducing peroxide compounds and peroxy acids with the use of a methanol-and- water solution of formaldehyde, and the reduced products are, next, esterified in the presence of a catalyst and methanol, and, next, the esterification mixture obtained is distilled, and the distillates produced are trans-esterified with the use of polyhydroxy alcohols.

Description

Method of manufacturing synthetic esters

The subject of the Invention is a method for manufacturing synthetic esters to be applied to produce biodegradable synthetic oils, lubricants, cutting liquids, and plasticisers.

A method is known from the description of the Patent No. GB1511038 (A); this method is developed to manufacture esters from the waste salt solutions derived from the production of cyclohexanone and includes the following processes: acidifying waste salt solution with sulphuric acid in order to obtain an aqueous phase and an organic phase; distilling water and low-boiling monocarboxylic acids from the organic phase to a temperature of 120°C; cooling the products left over after the distillation to a temperature between +50 to -10°C; separating crystals of solid bicarboxylic acid that precipitates during cooling; and fractioning a liquid left over after the crystals are removed for the purpose of removing more volatile components, i.e. hydroxy acids and bicarboxylic acids. Other acids that are left over in the liquid can be esterified using lower (as it has been defined) aliphatic alcohols; then, the product undergoes a distillation process for the purpose of separating more volatile esters from hydroxy acids and leftovers, which contain bicarboxylic acid esters. Preferably, the leftover after the second stage of distillation can be esterified using lower (as it has been defined) aliphatic alcohols. The manufacturing process as presented above results in the production of mono- or bicarboxylic acid esters.

In the description of the Polish Patent No. 141736, a method is disclosed that is applied to produce a methyl ester of ε-hydroxycaproic acid; according to this method an aqueous solution, formed during the washing of a raw product of cyclohexane oxidation process with water, is compressed at atmospheric pressure; next, it is again compressed at a pressure of 3 to 30 kPa, and, thereafter, it is esterified at, for example, an increased pressure during a liquid phase with the use of excess methanol. The subsequent process step is to distillate the esterified mixture produced at a decreased pressure. The distillation result is a ε-hydroxycaproic acid methyl ester and bi-methyl esters of butanedioic (succinic), glutaric, and adypic acids, all of which are applied as raw materials for further processing, in particular for producing synthetic fibres.

The description of the Patent No. EP2244996 represents a method to manufacture at least one 6-hydroxycapronic ester from a mixture of by-products resulting from the oxidation process of cyclohexane to cyclohexane and cyclohexanone. The method disclosed consists of the following stages:

oxidizing cyclohexane with molecular oxygen or with a mixture of molecular oxygen and gases; the product of the cyclohexane oxidation reaction is a mixture and the major components thereof are: cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone, unreacted cyclohexane, 6-hydroperoxide caproic acid, 6-hydroxycaproic acid, 5-formyl pentanoic acid, and (alpha) (omega) bi-carboxylic acids containing four to six carbon atoms;

adding water to the reaction mixture obtained during Stage 1 as above and separating it into a phase with cyclohexane and cyclohexane compounds and an aqueous phase with carboxylic acids;

catalytic hydrogenation of the aqueous phase obtained during Stage 2;

a reaction between carboxylic acids, present in the aqueous phase, and the alcohol containing from 1 to 10 carbon atoms for the purpose of producing suitable carboxylic esters along with the manufacture of an esterification mixture;

distillation of esters to separate them from the 6-hydroxy caproic acid in the esterification mixture obtained at Stage 4.

In the method as disclosed in the Patent No. EP2244996, the process of oxidizing the cyclohexane as shown in the description of Stage 1 above can run in the presence of a catalyst or without a catalytic converter; the aqueous phase with carboxylic acids from Stage 2 can be extracted using aromatic hydrocarbons in order to remove the remaining cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone, or it can be compressed by means of water distillation, whereas both the 6-hydroperoxide caproic acid and the 5- formyl pentanoic acid are hydrogenated to a 6-hydroxy caproic acid as referred to in the description of Stage 3. The methods as cited above indicate that the utilization of by-products formed during the cyclohexane oxidation reaction is rather limited; yet the method according to the Invention makes it possible to rationally plan and carry out the utilization thereof in the production of synthetic oils, lubricants, cutting liquids, plasticisers, and polyester polyols.

It is generally known that the reaction of oxidizing cyclohexane is a multistage, highly exothermic process with a number of after-reactions, i.e. the product from one reaction becomes a starting material for the next step in the reaction sequence. Cyclohexanol and cyclohexanone are the basic oxidation products. At the same time, during the oxidation process, a number of side-reactions take place and the oxidation products become transformed; as a result the final mixture obtained contains not only the required products, but, also, various amounts of oxidation products, such as: alcohols, aldehydes, ketones, acids, hydroxy acids, keto acids, esters, and, of course, water. The cyclohexane oxidation process runs during the aqueous phase by means of a gas dispersed therein and containing oxygen (usually air). As a result of the oxidation reaction, two streams leave the reactor: 1) a liquid stream (which is also called a post- reaction mixture) that contains oxidation products including water and unreacted cyclohexane; and 2) a gaseous stream containing inert air gases and unreacted oxygen, water vapour as well as evaporated cyclohexane and the products of its oxidation.

The objective of this Invention is to fully utilize the liquid stream formed during the cyclohexane oxidation process, since in this stream, in addition to cyclohexanone and cyclohexanol, there are contained cyclohexane oxidation products, which have the following acids, their amount being from 70 do 90 % by weight, preferably 80% by weight: adipic acid, 6-hydroxy caproic acid, caproic acid, glutaric acid, succinic acid (systematic name: butanedioic acid), acetic acid (systematically named ethanoic acid), 4-hydroxybutyric acid, butyric acid, isobutyric acid, formic acid (also called methanoic acid), propanoic acid (systematic name: propionic acid), valeric acid (or pentanoic acid), 5-hydroxy valeric acid, oxalic acid, malonic acid (systematic name: propanedioic acid), as well as peroxides (including cyclohexyl hydroperoxide), γ-butyrolactone, trace amounts of cyclohexanone and cyclohexanol, 1,2- cyclohexanodiol, 1,4-cyclohexanodiol and other substances in the form of 6-hydroxy caproic acid esters, cyclohexanodiol- esters, lactone-esters, and cyclic compound condensation products, as well as water amounting from 10 to 30% by weight, preferably 20 % by weight.

At the beginning, the liquid stream of the cyclohexane oxidation products undergoes a reduction process of peroxide compounds and peroxy acids with the use of methanol- and-water solution of formaldehyde, their per cent content 'by weight' ratios being: 45- 65% of water; 5-25% of methanol, and 30-50% of formaldehyde, preferably 50% of water, 10% of methanol, and 40% of formaldehyde. This reduction process is performed in a pressurized periodic reactor equipped with a coil pipe, a propeller mixer, and a bubbler (designed to presumptively bring in a neutral gas, preferably nitrogen), at a temperature ranging from 20 to 100°C, preferably 80°C, and at a pressure between 0.05 and 0.3 MPa, preferably 0.2 MPa.

The duration time of the reduction process of peroxide compounds and peroxy acids is from 6 hours until the reaction is stopped, i.e. until the peroxide value (PV) yielded does not exceed 3% by mass, preferably PV does not exceed 0.5% by mass. 6 hours after the continuation of the process, a first sample is taken and analyzed for the purpose of determining the content of peroxide compounds therein. A second sample is taken 2 hours after the further proceeding with the process and, again, it is analyzed in order to determine the content of peroxide compounds. The samples analyzed are compared with each other. In the case the difference between the contents of peroxide compounds in the samples analyzed is lower than 10%, preferably 5%, the reduction reaction is considered as finished; in the case those differences exceed 10%, the reduction process is continued for another two (2) hours, then, a third sample is taken, analyzed, and compared with the previous samples to check the differences among them. The peroxide compounds are periodically analyzed until the entire reduction reaction is accomplished; thereafter, the overpressure is reduced to a value of atmospheric pressure and the process is continued at a temperature ranging between 40°C and 80°C, preferably 64°C until the whole amount of formaldehyde is removed from the mixture.

The next step in the method according to the Invention is an esterification process of reduced products of cyclohexane oxidation; preferably, the esterification process is preceded by the initial dewatering of those products by means of distillation until the water content therein is below 5% by weight. The reduced and dewatered cyclohexane oxidation products are esterified at a pressure ranging from 0.01 to 1.0 MPa, preferably from 0.1 to 0.6 MPa, and at a temperature ranging from 60 to 200°C, preferably from 80 to 140°C. The esterification process is performed in a pressurized periodic reactor, in the presence of a catalyst and methanol that is fed into the reactor and continuously replenished during the ongoing process until the yielded level of an acid value of the esterification product is between 2 and 15 mg KOH/g, preferably less than 5 mg KOH/g. The amount of methanol being brought in is by 0.5 to 3 times higher than the amount of the cyclohexane oxidation products, preferably twice as high as the amount of the cyclohexane oxidation products. Homogeneous or heterogeneous^' catalysts are applied to this process, among other things: sulphuric (VI) acid, hydrochloric acid, >-Toluenesulfonic acid (PTSA) or tosylic acid (TsOH), phosphoric (V) acid, Lewis acids such as aluminium, vanadium, titanium and boron compounds, as well as metal oxides, silicates and zeolites, their amounts ranging from 0.1 to 3% (preferably between 0.5 and 1.5%) of the mass of the reaction mixture.

The process continues until the acid value yielded is lower than 15 mg KOH/g, preferably 5 mg KOH/g; a first sample is taken 6 hours after the required working pressure has reached a level of 0.6 MPa and this sample is analyzed in order to determine the acid value. A second sample is taken after a 2 hour period of the process in progress and, again, the acid value is determined. The process carries on until the required acid value is yielded.

A mixture, obtained from the esterification process, is distilled to separate several fractions: the first fraction consists of the excess methanol and is distilled at atmospheric pressure, at a temperature that is gradually increased from 40 to 100°C. As soon as the excess methanol is removed, a process of separating the second fraction begins by graduating the vacuum from 33 to 600 hPa, preferably from 50 to 150 hPa, and, as soon as a 600 hPa value is achieved, the increase in the temperature starts and continues from 80 to 90°C until the end of the separation of the second fraction consisting of esters of mono-carboxylic acids. Once the extracting of the second fraction phase from the whole solution has been accomplished, the vacuum is increased from 20 to 60 hPa, preferably from 30 to 45 hPa; the latter is maintained during a period from 30 to 45 min. During this period, the intermediate fraction is extracted and returned to the subsequent esterification batch. The step of extracting the second fraction is carried on in order to avoid that the esters of monocarboxylic acids are mixed with the esters of di- carboxylic acids. Upon the completion of the extraction of intermediate fraction, the process temperature is increased from 110 to 150°C, preferably from 125 to 140°C and the third fraction is extracted; it consists of esters of di-carboxylic acids and esters of hydroxy acids. The distillation leftovers constitute the fourth fraction that is returned to the esterification process or it is utilized to manufacture special polyester polyols.

The second fraction is mainly composed of the following:

1. methyl formate (methyl methanoate), its amount ranging from 0.01 to 1,5%;

2. methyl acetate (methyl ethanoate), its amount ranging from 0.01 to 1.5%;

3. methyl propionate, its amount ranging from 0.01 to 1.5%;

4. methyl isobutyrate, its amount ranging from 4 to 12%;

5. methyl butyrate, its amount ranging from 1 to 5%,

6. methyl isovalerate, its amount ranging from 5 to 10%;

7. methyl valerate, its amount ranging from 20 to 60%;

8. methyl capronate, its amount ranging from 10 to 20%;

The third fraction is mainly composed of the following:

- dimethyl oxalate, its amount ranging from 0.01 to 1.5%;

dimethyl malonate, its amount ranging from 0.01 do 1.5%;

- dimethyl succinate, its amount ranging from 1.5 to 3%;

- dimethyl glutarate, its amount ranging from 2 to 6%;

- dimethyl adypate, its amount ranging from 30 to 60%;

- methyl 4 -hydroxy butyrate, its amount ranging from 0.01 - 1.5%;

- methyl 5 -hydroxy valerate, its amount ranging from 15 - 30%;

- methyl 6-hydroxy capronate, its amount ranging from 30 - 60%.

The fractions obtained from the distillation process constitute a raw material for further production. And so, the second fraction constitutes a basis to produce bio-degradable synthetic oils, lubricants, cutting liquids, and plasticizers for polyvinyl chloride (PVC). The third fraction is used to manufacture polyester polyalcohols and plasticizers for polyvinyl chloride (PVC). The choice what fraction should be selected for further processing depends on what synthetic ester is necessary for particular applications and, thus, has to be manufactured. For example: Fraction II serves as a basis for the manufacture of synthetic esters, which may be utilized, depending on the components used in the trans-esterification process, as synthetic, bio-degradable ester oil, cutting liquid, lubricant, or plasticizer, whereas Fraction III mainly serves to manufacture polyester polyols, and, moreover, it is a basis of ester plasticiser for polyvinyl chloride (PVC).

The subsequent stage of the method according to the Invention is a trans-esterification process of a mixture made from esters and poly-hydroxy alcohols, such as mono- ethylene glycol, diethylene glycol, or triethylene glycol, or trimethyl propane, or pentaerythritol, or dipentaerythritol.

The trans-esterification reactions are carried out in a periodic reactor, in the presence of acid catalyst, such as, preferably, compounds of aluminium, vanadium, titanium, or chlorine. The content of the catalyst applied ranges from 0.01% to 3%, preferably from 0.03% to 0.5% of the mass of the reaction mixture. The trans-esterification process is performed at a temperature of 60 to 200°C, at a decreased pressure between 20 hPa and 1013 hPa. This process continues until the acid value decreases below 2 mg KOH/g, preferably to a level of 0.2 mg KOH/g. Other parameters, such as viscosity and density, have a resultant character since they depend on the composition of the fraction used in the trans-esterification process.

Therefore, depending on the type of fraction utilized in the trans-esterification process and on the kind of poly-hydroxy alcohol, and based on the method according to the Invention, we obtain, for example:

- polyester polyols used in the manufacture of shoe soles; this compound consists of Fraction IV and poly-hydroxy alcohols; this polyester polyols is characterized by a hydroxyl value of 68 mg KOH/g, acid value of 1.2 mg KOH/g, water content of 0.05 % by weight, and viscosity equalling 11800 mPas at 35°C;

- polyester polyalcohol used in the manufacture of shoe soles, cast elastomers; this compound consists of Fraction III and di-hydroxy alcohols; it is characterized by an acid value of 0.15 mgKOH/g, hydroxyl value of 47 mg KOH/g, and viscosity of 803 mPas at 75°C; ester plasticizer for industrial lubricants and polyvinyl chloride (PVC); it consists of Fraction III and long-chain alcohols and is characterized by an acid value of 0.37 mg KOH/g, density of 0.988 g/cm at 20 C, water content of 0.02% by weight, viscosity of 55 mPas at 25°C, content of volatile substance of 0.32% by weight, at 100°C, and hazen color index of 150;

- plasticizer or bio-degradable synthetic oil consisting of Fraction II and polyhydroxy alcohols; it is characterized by an acid valued of 0.19 mg KOH/g, kinematic viscosity of 27 mm²/s at 40°C, water content of 0.013% by weight, density of 0.992 g/cm² at 20°C, and hazen color index of 125;

- complex esters for bio-degradable synthetic oils and lubricants; they consist of Fraction II (20% by weight), Fraction III (80% by weight), and di -hydroxy alcohols, and they are characterized by an acid value of 0.17 mg KOH/g, density of 1.062 g/cm³, kinematic viscosity of 60 mm /s, and water content of 0.023% m/m;

- bio-degradable synthetic ester that consists of Fraction II and polyhydroxy alcohols and is characterized by an acid value of 0.13 mgKOH/g, kinematic viscosity of 18 mm²/s at 40°C, water content of 0.013% by weight, density of 0.962 g/cm² at 20°C, and hazen color index of 100.

Example 1

In the pressurized reactor, 15000 g of the liquid stream of cyclohexane oxidation products is placed; in the liquid stream the amount of peroxides is 3.4 % by mass, the water content is 18.1% by mass, and the acid value is 321 mg KOH/g; next, 400 g of the water-methanol solution of formalin is added. The quantity of the solution is selected based on the initial determination of the peroxide amount and so as to make the molar ratio between the formic aldehydes and the peroxides per cyclohexyl hydroperoxide be more than 1. Thus, the composition of the mixture of raw materials is: cyclohexane oxidation products: 97.4% by mass, water-methanol solution of formalin: 2.6% by mass. Now, the mixture is heated up to ca. 80°C and, at the same time, the pressure is decreased to 0.12 MPa. The reduction process continues for 10 hours until the amount of peroxides obtained equals 0.63% by mass, and, at this point, the temperature is decreased to ca. 64°C, the pressure is decreased to the level of atmospheric pressure, and the nitrogen is supplied at a flow rate of 25 litres per minute. The reduced products of cyclohexane oxidation are heated up, one after the other, to a temperature of 140°C, whereas the temperature is adjusted so as to not exceed 100°C at the top of the column. The dewatering process is finished once the water content of 1.14% by mass has been achieved. The dewatered products of cyclohexane oxidation are characterized by an acid value of 390 mg KOH/g. They constitute a feed for the esterification process.

The reactor is fed with 5000 g of dewatered products, 10000 g of technical methanol, and 150 g of sulphuric (VI) acid of a 96% concentration level. The reaction mixture is heated up to 100°C and the pressure of 0.4 MPa is obtained. The esterification reaction continues for 11 hours until the acid value obtained is 15.7 mg KOH/g and the water content obtained is 3.96 % by mass; thereafter, the mixture if cooled to a temperature of 80°C, and the overpressure is decreased to the level of atmospheric pressure. Under the conditions of this ongoing esterification process, a condensate is produced with a water content of 4.5% by mass. Next, without changing the temperature, 5000 g of methanol is supplied into the reactor while the reaction continues for another 5 hours until a reaction mixture is obtained that is characterized by an acid value of 4.5 mg KOH/g and a water content of 2.5% by mass. After that, the post-reaction mixture is cooled to a temperature lower than 50°C, whereas the pressure is decreased by removing the excess methanol until the water content obtained in the reaction mixture is 0.41% by mass. The excess methanol obtained is Fraction I, i.e. the first of the fractions obtained during the distillation process. Provided the process runs according to this method, the process yield is a condensate containing a water amount of 4.5% by mass and constituting ca. 65% by mass of the feed. As soon as Fraction I is distilling off the mixture, the nitrogen is brought into the remaining mixture, and, at a temperature of 80°C, the graduation of the vacuum begins to get a level of 35 hPa and Fraction II is distilling off; Fraction II is ca. 15% by mass of the total feed.

The next step involves increasing the temperature, gradually, to 140°C. The intermediate fraction is distilling off a temperature between 80 and 90°C and within a time period of 30-45 minutes; this intermediate fraction constitutes ca. 0.4% by mass of the total feed. During the gradual rising of the temperature to 140°C, Fraction HI fraction is distilling off; its amount is ca. 17% by mass of the total feed. Fraction IV is a distillation leftover; its amount is ca. 2.5%» by mass of the total feed. Fraction IV, obtained using this method, is planned to be utilized to synthesise polyester polyols for the manufacture of black shoe soles. For this purpose the following is added to 1200 g of the fourth fraction: 46 g of monoethylene glycol, 8 g of trimethylpropane, and 0.05 g of Tyzor TPT catalyst. The whole is trans-esterified, and the result yielded is a polyester polyols showing the following characteristics:

- hydroxyl value 68 mg KOH/g;

- acid value 1.2 mg KOH/g;

- water content 0.05 % by weight;

- viscosity at 35°C 11800 mPas.

Example 2

Example 2 represents the synthesis of a plasticizer for industrial lubricants and polyvinyl chloride; here, the equipment set is the same as in Example 1, and the entire course of the process is the same as in Example 1 , but only until the moment when the fractions are separated. In this Example, Fraction III is applied and the following ingredients are added to it, their amounts are given in the brackets: Fraction III, diethylhexyl alcohol ((33 kg), and Tyzor TPT catalyst (1.5 g). Once the trans- esterification has been accomplished, the resulting product obtained is an ester characterized by the following parameters:

- acid value 0.37 mgKOH/g;

- density at 20°C 0..88 g/cm³;

- water content 0.02% by weight;

- viscosity at 25°C 55 mPas;

- content of volatile substances at 100°C 0.32% by weight;

- hazen color index 150

Example 3

Example 3 represents the synthesis of complex esters to be used in producing biodegradable synthetic oils and lubricants; the equipment set used is the same as in Example 1, and the course of the process is the same as in Example 1, but only until the moment when the fractions are separated. In this case, there are applied: 300 g of Fraction II, 1200 g of Fraction III with 50 g of neopentyl glycol and 1.0 g of Fascat 4100 catalyst added. As soon as the trans-esterification process is accomplished, the product yielded is a complex ester characterized by the following parameters:

- acid value 0.17 mgKOH/g;

- kinematic viscosity at 40°C 60 mm²/s;

- content of water 0.023% by weight;

- density at 20°C 1.062 g/cm²;

- hazen colour index 70

Claims

Patent claims What is clamed is

1. A method to manufacture synthetic esters from a liquid stream formed during a cyclohexane oxidation process, whereas the liquid stream contains acids, peroxides, hydroxycaproic acid esters, esters of cyclohexaneodiols, lactones, products of cyclic compounds condensation, as well as water, wherein the liquid stream undergoes an initial process of reducing peroxide compounds and peroxy acids by a methanol-and- water solution of formaldehyde, and the reduced products are, next, esterified in the presence of a catalyst and methanol, and the mixture obtained from the esterification process is distilled; the distillates are trans-esterified with polyhydroxy alcohols.

2. The method according to Claim 1, wherein the process of reducing peroxide compounds and peroxy acids is carried out with the use of methanol-and- water solution of formaldehyde, and the by-weight contents of the constituents are: 45-65% of water; 5-25% of methanol, and 30-50% of formaldehyde, preferably 50% of water, 10% of methanol, and 40% of formaldehyde, at a temperature ranging from 20 to 100°C, preferably 80°C, at a pressure ranging from 0.05 to 0.3 MPa, preferably 0.2 MPa.

3. The method according to Claim 2, wherein the process of reducing peroxide compounds and peroxy acids is carried out from 6 hours until the reaction is stopped, i.e. when the peroxide value obtained does not exceed 3% by mass, preferably it does not exceed 0.5% by weight.

4. The method according to Claim 1, wherein the reduced products of cyclohexane oxidation undergo an esterification process, preferably preceded by the initial dewatering of those reduced products by means of a distillation method so as to obtain a water content therein below 5% by weight, and, wherein the esterification process runs in the presence of methanol and a catalyst at a temperature between 60 and 200°C, preferably from 80 to 140°C, at a pressure between 0.01 and 1.0 MPa, preferably from 0.1 to 0.6 MPa, where the amount of the methanol supplied is by 0.5 to 3 times larger, preferably twice as large, than the amount of the reduced products of cyclohexane oxidation, and the content of catalyst varies from 0.1 to 3%, preferably from 0.5 to 1.5%, in relation to the mass of the reaction mass.

5. The method according to Claim 4, wherein the esterification process is carried out at least 6 hours from the moment when the required working pressure has reached a level of ca. 0.6 MPa until the moment when the acid value obtained is lower than 15 mg KOH/g, preferably 5 mg KOH/g.

6. The method according to Claim 1 or to Claim 4, wherein homogeneous or heterogeneous catalysts are applied to the esterification process, preferably such catalysts as sulphuric (VI) acid, or hydrochloric, or /?-Toluenesulfonic acid (PTSA), or phosphoric (V) acid, or Lewis acids, or metal oxides, or silicates, or zeolites.

7. The method according to Claim 1, wherein the post-esterification mixture is distilled in order to separate fractions and that Fraction I is separated by distillation at atmospheric pressure and at a temperature that is gradually increased from 40 to 100°C, and, next, the graduation of the vacuum is started until the pressure value is between 50 and 600 hPa, preferably from 50 to 150 hPa, and, once the required pressure value has been obtained, the temperature is also increased to a level between 80 and 90°C until the extraction of Fraction II is finished, and, next, the vacuum is increased to a level between 30 and 60 hPa, preferably from 30 to 45 hPa, and the process temperature is also increased to a level between 110 and 150°C, preferably from 125 to 140°C, while Fraction III is distilling off;.

8. The method according to Claim 7, wherein after the extraction of Fraction II and after the increase in the vacuum to a level between 20 and 60 hPa, preferably from 30 to 45 hPa, and along with the temperature maintained at a level between 80 and 90°C, the vacuum is maintained for 30 to 45 minutes, and an intermediate fraction is distilling off; this intermediate fraction is then returned to the subsequent batch of the esterification process.

9. The method according to Claim 7 or to Claim 8, wherein Fraction I consists of the excess methanol, Fraction II consists of esters of monocarboxylic acids, and Fraction III consists of esters of di-carboxylic acids.

10. The method according to Claim 1, wherein the trans-esterification process of ester mixture is carried out with the use of polyhydroxy alcohols, in the presence of the acid catalyst, at a temperature between 60 and 200°C, and at a decreased pressure between 20 hPa and 1013 hPa, until the acid value decreases to a level below 2 mg KOH/g, preferably 0.2 mg KOH/g, and the amount of acid catalyst is between 0.01% and 3%, preferably from 0.03% to 0.5% in relation to the mass of the reaction mixture.

11. The method according to Claim 10, wherein the compounds of aluminium, vanadium, titanium, or chlorine are preferably used as a catalyst.